1. Field of the Invention
The present invention relates to a fuel cell making use of an electrochemical reaction to generate electric power, and to a method for the manufacture thereof.
2. Description of the Related Art
Fuel cells are devices for converting chemical energy directly into electric energy by interposing an electrolyte membrane between a pair of opposing first and second electrodes, supplying a fuel to the first electrode and an oxidizer to the second electrode, and permitting the fuel and the oxidizer to react inside the fuel cell electrochemically. Among such fuel cells, solid polymer fuel cells can achieve high output characteristics because they use solid polymer electrolyte membranes having proton conductivity for the electrolyte membrane.
The electrochemical reactions in these solid polymer fuel cells are sustained by continuously supplying a reactant gas. Ion conductivity of the electrolyte membrane, which plays an important role in these electrochemical reactions, is generally proportional to the water content of the solid polymer electrolyte membrane. In addition, because the water content of the solid polymer electrolyte membrane is dependent upon the humidity of the reactant gas, the reactant gas is humidified during operation. It is also necessary to seal off the gases flowing to the first and second electrodes so as to prevent leakage not only to exterior portions, but also between the first and second electrodes. Thus, because the solid polymer electrolyte membrane plays a role in preventing leakage of the reactant gas between the first and second electrodes, the solid polymer electrolyte membrane is extended to outer peripheral portions of an electrode portion so as to prevent a gas seal between the first and second electrodes from being compromised.
However, because the solid polymer electrolyte membrane contracts when it dries out and swells when it absorbs moisture, stresses act on the solid polymer electrolyte membrane with repeated running and stopping. In addition, if low-humidity air is supplied through an oxidizer supply port, the solid polymer electrolyte membrane dries in an upstream region of the airflow channel, and the solid polymer electrolyte membrane is wet by water produced by the reaction in a downstream region thereof. Because stresses act on the solid polymer electrolyte membrane in this manner, there has been a risk that the solid polymer electrolyte membrane may be damaged unless the solid polymer electrolyte membrane is reliably supported.
In addition, damage to the electrolyte membrane often occurs in portions of the electrolyte membrane positioned at a boundary between a power generating portion and a gas seal portion. One of the reasons that can be given therefor is deformation and concentrations of local stresses in the electrolyte membrane at the boundary between the power generating portion and the gas seal portion due to at least one side of the electrolyte membrane not being fixed. In particular, when the electrolyte membrane in this portion is exposed to wet reactant gas, damage to the electrolyte membrane occurs more easily due to stresses accompanying contraction, etc., of the electrolyte membrane. Another reason that can be given is concentrations of stress resulting from nonuniformity of water content in the electrolyte membrane. In other words, the electrolyte membrane is more likely to be wetted by the reaction product water in the power generating portion, and on the other hand, the electrolyte membrane is more likely to dry out in the gas seal portion.
Thus, if the electrolyte membrane cracks, or pinholes form in this manner, gas may leak through these between the first and second electrodes, reducing the output voltage of the fuel cell. In addition, damage to the electrolyte membrane may spread if operation is continued without intervention because the rate of gas leakage will increase, react on a catalyst, and partially increase the temperature. Thus, ultimately the fuel cell will cease to function.
Thus, in conventional fuel cells, portions of gas-permeable porous electrodes overlapping with a sealant may be impregnated by the sealant. (For example, see Japanese Patent Laid-Open No. HEI 8-45517.) Processes may also be applied to the solid polymer electrolyte membrane so as to enable the solid polymer electrolyte membrane to be reliably supported. For example, the solid polymer electrolyte membrane has: a current-carrying portion having a larger surface area than the first and second electrodes, contacting the first and second electrodes; and a non-current-carrying portion not contacting the first and second electrodes. A reinforcing material may be included at a boundary portion between this current-carrying portion and the non-current-carrying portion. A fluorine-containing polymer may be used for this reinforcing material. (For example, see Japanese Patent Laid-Open No. 2000-260443) However, stresses are concentrated at a boundary portion between the current-carrying portion of the solid polymer electrolyte membrane and the non-current-carrying portion reinforced by the reinforcing material, and there has been a risk that cracking may occur along that boundary portion.
Thus, an electrode substrate supporting the solid polymer electrolyte membrane may be reinforced so as to enable the solid polymer electrolyte membrane to be reliably supported. For example, the electrode substrate has a surface area identical to that of the solid polymer electrolyte membrane, and a peripheral edge portion of the electrode substrate is compacted by a compactant constituted by an adhesive. The solid polymer electrolyte membrane is supported by this compacted peripheral edge portion, and forms a gas seal. Compaction may be performed using a polytetrafluoroethylene dispersion for the compactant by impregnating the peripheral edge portion then removing a solvent. (For example, see Japanese Patent Laid-Open No. HEI 8-148170)
Another method for reinforcing the electrode substrate is to impregnate a sealing region surrounding the power generating region of the electrodes with an injection moldable thermosetting liquid compound and harden it. (For example, see Japanese Patent Publication No. 2001-510932)
Now, as disclosed in Patent Literature 3, compaction of the electrode substrate may be performed by impregnating the electrode substrate with a polytetrafluoroethylene dispersion functioning as a compactant, then removing a solvent. However, one problem has been that many communicating pores remain in a resin that has been hardened by performing impregnation then removing the solvent, making it difficult to seal off the fuel and the oxidizer gas. In addition, because the solid polymer electrolyte membrane is stacked after the electrode substrate has been compacted, another problem has been that it is difficult to seal the gases off completely since the surface of the solid polymer electrolyte membrane is merely in contact with and not bonded to the surface of the compacted portion of the electrode substrate.
As disclosed in Patent Literature 4, an integral seal may also be formed by impregnating a portion of an electrode layer of a membrane electrode assembly with an injection moldable thermosetting liquid compound as a sealant material and hardening it. However, since electrical conductivity and gaseous diffusivity are required of the electrode portions, porous carbon materials are generally used. For that reason, another problem has been that it is difficult to inject the liquid compound only in the sealing region of the electrodes with complete vacuum impregnation, and it also enters the electrochemically active region, reducing the area of the active region.